WLAN provides an Ethernet-like channel that uses wireless media (e.g., radios) instead of wires or cables to enable communication between or amongst computers and other types of electronic devices. While providing mobility and portability, WLANs avoid the effort and costs involved in running and maintaining cables, and thus are becoming increasingly popular as a communication medium. A WLAN can have multiple stations and access points (APs), where an AP serves to attach or connect respective stations to an external network and to one another. Generally, a WLAN has several protocol layers—among which are a physical (PHY) layer and a medium access control (MAC) sub-layer. A MAC, comprising hardware and software, is located at each station and AP, and controls access to the medium. A transmission from an AP to a station is referred to as a downlink, a transmission from a station to the AP is referred to as an uplink, and a transmission between stations is referred to as a sidelink.
Because all transmissions within a WLAN must share a single channel or communication medium, conflicts or collisions are likely to occur whenever different traffic streams and bursts simultaneously arrive at one or more transmission points (i.e., stations and the AP collectively) and need to access the shared medium for transport to other transmission points. Accordingly, several approaches have been developed to mediate access between competing traffic streams and bursts, and to provide access scheduling that avoids collisions over the medium.
In an approach commonly known as Distributed Coordinator Function (DCF), the station must sense the medium before a data frame is sent from a station. If the medium is found to be idle for at least a DCF Inter Frame Space (DIFS) period of time, the frame is transmitted. Otherwise, a back-off time B (measured in time slots) is chosen randomly in the interval [0-CW], where CW is the so-called contention window. After the medium has been detected as being idle for at least a DIFS, the back-off timer is decremented for each time slot the medium remains idle. When the back-off timer reaches 0, the frame is transmitted. Upon detection of a collision, a new back-off time is chosen using a CW that is double the previous one, and the back-off procedure starts over.
An enhancement of DCF, commonly known as EDCF, has been developed to provide service differentiation. With EDCF, once a station has gained access to the medium, it can be allowed to send more than one frame without contending for the medium again. More particularly, after obtaining access to the medium, a station is allowed to send as many frames as it wishes, as long as the total access time does not exceed a specified limit.
Furthermore, a WLAN environment may be fairly hostile to radio signals, due to noise and interference as well as a number of other disruptive factors. Accordingly, it is common practice to have a recipient acknowledge a transmission, and for a sender to resend a transmission upon failure to receive an acknowledgement signal after a period of time.
Moreover, WLAN transmissions may be of different traffic types including both isochronous streams and asynchronous bursts. Isochronous streams have quasi-periodic data arrivals and include voice and video streams. Asynchronous bursts are essentially traffic arrivals in burst with low duty cycles and include file transfers and interactive data. The streams and bursts can be of Continuous Bit Rate (CBR) or Variable Bit Rate (VBR), can have respectively different data rates, and can be different from one another in regard to data rates, delay and jitter requirements, acknowledgement policies and other parameters. Also, while there may be distinct advantages in using long data frames in transmissions, there may be other advantages in using shorter frames.
Because of the widely varying characteristics of WLAN traffic, conventional techniques for scheduling access to a WLAN, such as EDCF, have certain significant limitations. For example, if a station having a number of long data frames to send gained access to the medium in an EDCF arrangement, it could transmit its long frames until it was finished, while a station with a shorter frame was forced to wait. This would occur despite the desirability of interposing longer and shorter time frames during successive time periods, such as to reduce jitter between the adjacent longer frames. It might also be desirable to give priority to certain signals in scheduling access (e.g., to enable real-time critical data frames to be sent before non real-time critical data frames). Furthermore, EDCF is a contention based access method, resulting in heavy collisions due to simultaneous transmissions from stations as the number of contending stations increases. Although scheduling methods for contention-free transmissions exist, none of them appear to addresses shared medium access by downlink, uplink, and sidelink, as is the case in a WLAN.
As a result, there is a need for a system for scheduling contention-free access times for data transmissions in a WLAN according to selected Quality of Service (QoS) requirements, such as data rates and delay bounds.